Dynamic uniaxial compression testing of veined rocks under high strain rates

Abstract

The occurrence of strainbursts is of major concern in underground deep mining operations. Strainbursts are complex events characterized by a spontaneous strain-energy liberation at the boundaries of underground excavations, putting both the long-term safety of operations and exposing workers to risk. In this study, we reported a series of uniaxial compression tests by using a split Hopkinson pressure bar (SHPB) and a high-speed camera, to understand the role of dynamic loads and high strain rates (10–102 s-1) in the mechanical behavior and fracture patterns of veined rocks. The samples are representative of major deep underground mine districts in Chile: Chuquicamata (QS), Andina (RB), and El Teniente (CMET). Results show a clear increase in the dynamical uniaxial strength concerning quasi-static values. The increase was up to 2,2 times in CMET, 1,8 in RB, and 2,6 in QS. Additionally, the dynamic increase factor (DIF) indicates a strong strain rate dependency of the QS samples. This effect on RB and CMET samples is less evident. The data obtained with dynamic compressional testing indicates that both the increase in stiffness and dynamical compressive strength depends largely on the rock strain rate dependency. Further, we report a direct relationship between fracture patterns and the presence of veinlets. We suggest that fractures propagate through the matrix when the mechanical properties show a strong strain rate dependency, whereas are restricted to veinlets and veins when this dependence is less evident. Thus, as stress pulses enhance the stiffness, the energy dissipation increases, and the proneness of block formation is higher. These are conditions favoring strainbursts occurrence and might increase the severity of this phenomenon in underground excavations.